mbed

I find I need an address of 0x40 for the PCF857. I suspect the code example above was written before your change to 8-bit I2C addressing.

Encouraged by Simon, I've made the appropriate change to the code above.

Does this code also work for the 8574AP variant?

There is only a difference in the adressing between a normal pcf8574 en the a version.

I bought this and all is working fine. I have tried the write command and it works. But read command is not working is this correct ?

#include "mbed.h"
#include "PCF8574.h"

PCF8574 io(p9,p10,0x40);

int main() {
int a;
  while (1){
    
       if ( io.read() == 0x65)
        {
                
       io.write(0x64);
    }}
    
}

Basically if a button is pressed at 5th pin a led should glow at 4th pin.. I have tried connecting the other pin of the button to +3.3v and ground, but nothing seem to work.

Do you have pull-ups or pull-downs connected to the inputs of the PCF8574?

Lerche

You mean at the 5th pin ? Then no I have not connected any pull up or pull down. Do I have to connect ?

P.S. I have connected 2.5k pull up to SDA and SCL.

What do you mean by 5th pin?

The 5th pin of the chip, or the 5th bi-directional port-pin?

Please draw a simple schematic. That would be much easier to understand.

Yes its the 5th bi-directional port-pin

So if i press the button at P5, the led should glow at P4. I need to know what should be connected at A. Thanks

Here's the schematic I'd use.

Have you put pull-down/pull-up on the other pins as well?

This is probably necessary, as when reaqding the input, those pins must not hang. Put 100k pull-down on all of the inputs, then this should take care of it.

Lerche

You dont really need a pullup on input pins. The PCF8574 has internal weak pullups. However, you do need to write a '1' to every bit that you want to read. The weak pullup will then allow you to read a '1' when the switch is open or read a '0' when the switch is closed. When you have previously written a '0' to a port pin you will always read back a '0' no matter what the switch state is or whether or not you have an external pull up installed. I would also mask and test the specific bit on P5 rather that just compare with a hardcoded value. That way you can later add more switches or LEDs without changing the test.

if ( (io.read() & 0x20) == 0x00) {
// P5 switch pressed
}

Christian is right about the series R for the LED, you need that to limit the max current.

I've posted sample code and a video of a working circuit at http://romillys-robots.blogspot.com/2012/03/pcf8574-i2c-and-mbed.html

The breadboarded circuit is as given by Christian Lerche, with a resistor added in series with the LED but without the 100k resistors on the unused inputs.

Instant demo. Great!

Thanks it worked :)